Take a narrated 1.5 hour Mississippi Riverboat cruise. Only $13 if you book online. Not a bad deal, IMHO. Departs at noon and 2pm.
http://www.riverrides.com/Pages/publ...ly_cruise.html
Otherwise, if you like nature and walking around, consider the Minnesota Landscape Arboretum:
http://www.arboretum.umn.edu/
(In other words, if the weather cooperates, enjoy the outdoors...

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